A high-quality coating film with a uniform thickness is produced without crush of coating particles even in a case where a wet coating material that does not require a step of drying the coating material is used, while maintaining high productivity. By providing a surface layer with an optimized hardness on the surface of a roll used for supply of a mixture coating material, a high-quality coating film with a uniform thickness can be produced, even in a case where a wet coating material that does not require a step of drying the coating material is used, while maintaining high productivity.